l. Field of the Invention
The invention relates generally to an integrated semiconductor structure having an epitaxial semiconductor layer, divided into regions by isolation zones and containing the active and passive semiconductor devices, of a first conductivity type formed on a substrate of a second conductivity type. A reference potential, and first and second supply voltages are applied to the structure.
2. Description of the Prior Art
Such layered, isolated, integrated semiconductor structures are known to the prior art and contain specific devices and circuits integrated in the isolated regions.
In the selection of the operating voltage to be applied to the devices and circuits of such structures certain conditions have to be observed. One condition is obvious, i.e. to select the operating voltage in such a manner that the circuits work correctly. Other conditions require that isolation be provided between the individual circuits and devices and that this isolation be ensured under all operating conditions and that there be no inadmissible substrate currents. Taking these points into specific consideration the prior art structures will now be discussed with more detail. Generally, a P-conductive substrate is coated with an N-conductive epitaxial layer in which the actual devices and circuits are formed. In such a structure the isolation consists of highly doped, P.sup.+-conductive isolation zones passing through the epitaxial layer down into the substrate. Thus, the isolation of individual regions of the epitaxial layer is achieved laterally through the PN-junction isolation diodes formed by the P.sup.+-conductive isolation zones with the N-conductive epitaxial layer, and vertically, through the PN-junction formed between the N-conductive epitaxial layer and the P-conductive substrate. In order to ensure isolation of the individual regions the respective PN-junction diodes have to be reverse biased. This is accomplished in the prior art for example, by applying to the substrate a first supply voltage which is negative with respect to a reference potential applied to the P.sup.+-conductive isolation zones. As a supply voltage for the circuit itself the prior art uses a second supply voltage having a value between the reference potential and the first supply voltage. Thus, the first supply voltage applied at the substrate or the isolation zone, respectively, is more negative than the second supply voltage and assures that the isolating PN-junction diodes are reversed biased.
However, during turn on, the operation voltages, i.e. the first and second supply voltages, can rise with differing speeds. Thus, for example the second supply voltage may rise to its final value while the first supply voltage is still below its final value and may be insufficient to ensure reverse biasing of the isolation diodes. In such a condition the isolation diodes are forward biased and a short-circuit ensues. The short-circuit current thus flowing can destroy the semiconductor structure.
One method for avoiding this problem requires that the second supply voltage be applied only after the first supply voltage has reached its final value thus ensuring that the isolation diodes are reversed biased. This successive application of the voltages requires a highly complex current supply unit.
For some specific circuits it is possible to avoid this problem by providing a protective resistor between the substrate and the substrate voltage. This protective resistor limits the short-circuit currents. This solution is shown in the "IBM Technical Disclosure Bulletin", Vol. 11, No. 7, December, 1968, p. 866.
This latter simple solution cannot be applied in many modern semiconductor structures. Frequently, these modern semiconductor structures are complicated with lateral PNP transistors and vertical, saturated NPN transistors that save space, and thus increase the packing density of the circuits. Often these modern semiconductor arrangements, require for normal operation substrate currents caused by parasitic transistors. As these substrate currents vary considerably in their magnitude, inserted, protective resistors would cause uncontrollable voltage drops which could jeopardize the functioning of the arrangement.